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2016SummerShotcreteEMag

Shotcrete FAQs Question: I am a homeowner who is having a pool built in my backyard. The company used shotcrete last Thursday, but didn’t tell us we needed to keep it wet for the next few days. We found out on the following Monday that we should have been keeping the shotcrete wet. The 4 days that passed before we began wetting the shotcrete were very windy and hot (temperatures in the low 80s °F). The pool company is now telling us that it’s probably not a big deal that the shotcrete wasn’t kept wet for 4 days. My question is this: How has the shotcrete been compromised by not keeping it wet for 4 days? What can I expect to happen to the shotcrete (cracks?) What would you recommend as far as a fix? Answer: ASA recommends a minimum of 7 days curing to help control shrinkage issues in young concrete sections. Lack of curing, and exposure to windy, hot, or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength. However, shotcrete generally exceeds the minimum 4000 psi (28 MPa) 28-day compressive strength ASA recommends, and required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604, “Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete,” gives guidance on taking cores from existing structures. A minimum 3 in. (76 mm) diameter core is recommended. Before coring, it is recommended to use ground-penetrating radar (GPR) or similar equipment to identify the location of reinforcement in the pool section, and then take cores to avoid cutting through the reinforcement wherever possible. The core holes would then need to be filled with a high-strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions. Question: A client of mine has requested an upgrade to their mine conveyor entrance, which is currently formed from natural earth and was initially covered with a geotextile. The wind has continuously blown the textile off and as a result the slope has kept eroding. There are areas of surface undulations but the general surface is sound. They have now requested a shotcrete solution. The entrance is a V-shaped valley with a conveyer running at the bottom. Slopes on either side are 1:1 and extend in a series of levels each about 33 ft (10 m) high. Each level has an interceptor ditch. Slope stabilization is not of concern but rather erosion control. I will be using polypropylenereinforced fiber shotcrete. I have a few questions: • What thickness of shotcrete would be optimal over the large areas to prevent further erosion for a long service period of 10 years? The shotcrete is to act as a barrier and not to stabilize the surface specifically. • How would I anchor the shotcrete onto the soil? Types of anchors and spacing? Is it possible to anchor onto soil without a type of surface preparation? • At what spacing would joints need to be installed? Answer: These are all good questions. Shotcrete is an excellent solution for the proposed upgrade. However, these are really design questions that should be evaluated by a licensed professional engineer experienced in slope stabilization, soil nails, and shotcrete. You may want to consult our Buyers Guide for contacts with our consultant members: www.shotcrete.org/ pages/products-services/Buyers-Guide/index.asp. Although most of our members are in North America, several members consult on projects around the world. Question: Can you please provide me a technical recommendation on whether or not expansion joints should be used in a large shotcrete pool that is approximately 230 x 135 ft (70 x 41 m)? In my design I am calling for two expansion joints, which would break the pool into three approximately 75.5 ft (23 m) sections. The contractor is telling me that he typically does not use expansion joints in the pool and that they are unnecessary. I do not typically work with shotcrete and have limited pool design but given the size of the structure I would think it would be best to include expansion joints. Can you please recommend whether or not the expansion joints should be used? Any help would be greatly appreciated. Answer: Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Pools will also experience volume change in the concrete due to thermal changes, especially summer to winter seasonal swings. Contraction and expansion joints are common in all kinds of concrete liquid-containing structures, especially with walls of this length. Although we can’t provide a firm design recommendation, you should consider these factors: • What are the weather conditions when the pool is anticipated to be built? If during hot summer months, could there be enough seasonal temperature swing to require expansion joints? • If expansion joints wouldn’t be needed, would contraction joints be needed to handle anticipated temperature swings and drying shrinkage? • Will the pool be empty for extended times? (This could lead to more shrinkage or direct exposure to solar gain or cold conditions.) • Is the pool to be kept full or empty during the winter months? (If the pool is in a geographic region where extending freezing conditions are prevalent…) Overall, the design for a shotcrete pool should be the same as one for a cast concrete pool. Shotcrete • Summer 2016 79


2016SummerShotcreteEMag
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